Remember that the ATAS-120 is a (shortened) 1/4-wavelength vertical antenna, and its counterpoise (artificial ground) is very important in accomplish a successful tuning exercise.

Auto-tuning antennas like the ATAS-120 utilize some fixed capacitors across a base matching coil, and their values are based on the use of a "normal" counterpoise that can emulate a 1/4-wavelength radial system; radical departures from this, like a flagpole that resembles a long fat wire, tend to be very resonant on one band, but may present a high impedance on other bands. This will make tuning virtually impossible.

The ATAS-120 works best when securely bonded to a vehicle body via a mount that bites into the metak frame of the vehicle. If the counterpoise system is significantly worse than this kind of design, think in terms of improving the artificial ground by installing some radials for the various bands (if you are in a fixed-station environment).

You probably have Menu #31 ([TUNER]) disabled, or set to the "Tuner" option. Set it to the "ATAS-100" option and you will be set.

This advice can apply to any radio supporting the ATAS-100/120 series of antennas: you must have the Menu for the radio properly engaged, so that the radio knows it is supposed to be looking for an antenna, and not an antenna tuner!

No. Automatic tuning (and RF output!) on the 3.5 MHz band are disabled when the ATAS-120 is selected via the radio's Menu. There is no work-around to allow you to fool the radio into trying to tune the lengthened antenna.

Generally, mag-mounts do not work well, for a couple of reasons. For one, the wind area of the ATAS-120 is considerable, and if you drive at high speed you may have the mount fall over, causing damage to the car exterior. Secondly, mag-mounts do not bond to the car body, and erratic tuning, erratic operation (RF feedback) and/or losses may occur if a secure bond is not made to the car body.

In the absence of a suitable counterpoise, the RF energy starts trying to create one itself, and the shield of the coax is the first place that it tries. If current is set up on the shield of the coax, it may get into the radio itself, causing distortion of your audio, improper switching, and erratic antenna tuning.

But in the world of antennas, sometimes the least-likely-to-succeed antennas do work, to the foregoing is not to say that all mag-mount systems will not work. Just be prepared to run a ground strap to the vehicle body to establish a mechanical bond, and monitor the integrity of the magnetic mount's holding power.

The first thing you should always try is to choke off any current tending to flow on the shield of the coaxial cable feeding the ATAS-120. Try making a coil of about eight turns, about 4" in diameter, as close as reasonably possible to the feedpoint (where the antenna mounts). If this does not solve the problem completely, try adding another such choke as near as possible to the radio. Just wind the coils up and tape them together to hold them in place. This actually is a very rigorous solution to this problem, if the feedback is related to current flowing on the coax.

If that doesn't work, try installing snap-on ferrite cores onto the microphone and any connected remote-control cables between the transceiver body and the remote head; the microphone cable should be the first one you work on.